1. Field of the Invention
The invention concerns the process of molding plastic material ferrules for optical fiber connectors.
Ceramic ferrules can be machined very accurately while are costly but plastics material ferrules, although much less costly, are often mediocre in terms of dimensional accuracy.
2. Description of the Prior Art
Conventional techniques for molding plastic materials (both thermoplastic and thermosetting materials) are rarely able to achieve the necessary accuracy: a typical ferrule is cylindrical, 12 mm long with an outside diameter of 2 500 .mu.m and an axial bore with a diameter of 127 .mu.m; both diameters must be accurate to with (0, +1 .mu.m) and the maximum permissible offset between the centers of the inside and outside diameters is 0.7 .mu.m.
The mold therefore comprises a hollow cylindrical member (sometimes called a "precision bush") which is accurately machined and which receives an axial peg carrying at its end an accurately sized pin with a diameter of 126 .mu.m (0, -1 .mu.m). The bush and the needle can be made very accurately, but there remains the problem of making them concentric to within better than 0.7 .mu.m. Because of molding constraints the peg carrying the needle is joined to the cover of the mold and it is therefore not possible to fasten the needle and the bush together as they have to be separated during each molding cycle. This makes it more difficult to achieve the required concentricity.
To overcome this problem, it has previously been proposed to use a "backing member" which is a tight fit in the bush and which has a 127 .mu.m diameter bore which receives the needle and positions it accurately when the mold is closed and during injection of the plastic material (guide function).
However, a drawback of this backing member is that it impedes the evacuation of the gases contained in the molding cavity during injection (vent function). Given the constraints associated with molding bushes, venting via the mold closing plane in the usual way is not possible. Also, the clearance between the 126 .mu.m needle and the 127 .mu.m bore in the backing member is insufficient to provide a vent because the head loss in any such interstice would be much too large, in particular as the residual gases must be removed in a very short time period, the injection of the plastic material taking only a few seconds (in some instances less than two seconds).
For reasons to be explained hereinbelow, techniques of this type previously proposed do not constitute an entirely satisfactory solution with respect to venting and guidance. Until now it has not been possible to arrive at a compromise between large clearances or tolerance build-up which are detrimental to the accuracy of guidance and small clearances which prevent effective venting.
An object of the invention is to propose a backing member geometry which reconciles the requirements of the guide and vent functions and makes it possible to achieve the required concentricity of the inside and outside diameters of the molded ferrules.